The invention relates to a looper drive for a chain stitch sewing machine having at least one needle and a looper mounted on a looper shaft. The sewing machine of this invention is of the type that includes a main rotary drive shaft with cams secured thereto. A first end of a pull rod surrounds a first cam, carried by the main rotary drive shaft, and the other end of the pull rod is drivingly connected to a swivel shaft that is disposed perpendicular to the main shaft. The swivel shaft has a first drive arm that is connected to said other end of the pull rod and a second drive arm that is connected through a flexible connection to the looper shaft. The swivel shaft imparts reciprocating sliding drive to the looper shaft. The main rotary drive shaft has a second cam secured to it that is surrounded by a yoke that imparts swivel motion to the looper shaft.
U.S. Pat. No. 3,460,494 discloses a sewing machine with a looper drive in which a rotating shaft has a cam secured thereto which is surrounded by a follower which is movable perpendicular to the main shaft. The follower is flexibly connected by means of a pull rod to one arm of a crank arm which is pivotally mounted on the sewing machine housing. The crank arm is joined through its other arm by means of a swivel connection consisting of a spherical bearing to the looper shaft, which causes reciprocal movement of the looper shaft. The swivel or oscillating movement of the looper shaft is induced by means of a second cam on the rotating shaft that is surrounded by a yoke connected to the looper shaft.
Although, induction of the thrust motion to the looper shaft through the crank arm and the spherical bearing results in a simple looper drive construction, at high stitch rates, e.g., 3500 r.p.m., this swivel connection causes vibration and premature wear. The vibrations and excessive wear is a result of the mechanism for reciprocating the looper shaft. The looper shaft is reciprocated by a crank arm that rocks about a fixed pivot axis. The free end of the crank arm is connected through a spherical joint to the looper shaft for imparting reciprocating motion to the looper shaft. However, this drive also imparts an undesirable and unnecessary component of movement to the looper shaft which, especially at high speeds, renders the drive unacceptable. Furthermore, prior art looper drives including swivel connection such as this produces approximately equal looper path speeds in the region of the front or left and rear or right end position of the looper. This is disadvantageous in the formation of a chain stitch, especially in a sewing machine in which the needle lift is approximately 60 millimeters and the looper travels approximately 30 millimeters.
For the foregoing reasons, there is a need for a sewing machine having a looper drive that will not cause excess vibration or wear and can produce a better quality chain stitch.